The present invention relates generally to motor drive systems and more particularly to motor drive systems using two what are commonly referred to as six-pulse inverters to provide twelve-pulse operation for an alternating current (ac) motor.
The advantages of employing a twelve-pulse drive, as opposed to the more common six-pulse drive, for the operation of a large ac motor are generally well known. Such twelve-pulse systems normally operate with less harmonics which are reflected back on to the power supply lines to affect other connections to those lines. In addition, insofar as motor operations are concerned, the motor torque pulsations are normally less severe in a twelve-pulse system since these pulsations are of twice the frequency of those of a six-pulse system and their amplitude on a percentage basis is approximately one-half that of the six-pulse system.
Perhaps the most important reason for the use of a twelve-pulse system, however, is that by using, what is here termed a double three-phase motor, higher operating speeds can be obtained. Such double three-phase motors have been known in the art for a number of years and include two sets of stator windings which are phase displaced from each other by some fixed amount, normally thirty electrical degrees. A teaching of such a motor can be found in U.S. Pat. No. 3,611,085, "Preventing Torque Pulsation in Inverter-Fed Polyphase" by George L. Rosenberry, Jr., issued Oct. 5, 1971 and assigned to the assignee of the present invention. This patent, which is hereby specifically incorporated by reference hereinto, shows two voltage source six-pulse inverters supplying what is here being called a double three-phase motor. Although the showing in this patent is that of two delta windings phase displaced, it is apparent that wye winding configurations could be employed with equal facility.
In supplement to the above advantages of twelve-pulse motor operation as such, it is sometimes desirable to utilize the same control for the seqential operation of both three-phase and double three-phase motors. An example of such usage is in dragline operation where double three-phase motor is used for hoisting operations but a standard three-phase motor is employed for the propel function in order to hold to a minimum the number of power slip rings required in supplying the propel motor. Since the hoist and propel functions are not simultaneously employed, the desirability of having a single control for the two types of motors is readily apparent.